fix(asr): use content_dir for subtitle output instead of video_path

- Add content_dir field to ASRInput schema
- Pass content_dir from pipeline when creating ASRInput
- Modify _try_official_subtitle to use content_dir for output
- Modify _run_whisper_basic to use content_dir for output
- Add validation for empty video_path in Whisper fallback

Root cause: DownloadProcessor sets content_file="" for video types,
causing Path("").parent to resolve to current directory instead of
content directory. Subtitle files were written to wrong location.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

kb_pipeline/pipeline.py

@@ -185,11 +185,13 @@ class Pipeline:
                     # ASR needs video path from download step
                     download_result = results.get("download", {})
                     video_path = download_result.get("content_file", "")
+
                     step_input = ASRInput(
                         content_id=parsed.content_id,
                         platform=parsed.platform,
                         video_path=video_path,
                         content_type=parsed.content_type,
+                        content_dir=str(content_dir),
                     )
                 elif step_name == "analyze":
                     # Analyze needs metadata and content files